iOS not connecting to new MacBook Air - Sequoia

I've just upgraded to a new MacBook Air from an older one. The iPhone can still connect to rekordbox on the old MacBook Air, but not the new one. Apparently, if I'm not mistaken, we can register up to three devices - but I can't find how to - and de-register the old Mac (but can't find how to). Any pointers please? It's probably worth saying I transferred all my files over from the old Mac, including Rekordbox - so I never installed a new version - and I've since deleted the imported version and installed a newer one, yet all the files/playlists/settings were somehow found and remain the same, and can't register the 'new application'.